In order to achieve high data rates, several technologies including Evolved Universal Mobile Telecommunications System (UMTS) terrestrial radio access (EUTRA) and EUTRA network (EUTRAN) have been developed in the Third Generation Partnership Project (3GPP) Long Term Evolution (LTE). Furthermore, local service requirements have led to the development of newer technologies in LTE-Advanced and are being proposed for “5G” implementations.
In order to provide local services, one approach is to use license exempt spectrum of wireless local area networks (WLANs). Another approach is data transmission on a licensed band in a coordinated and planned network. Toward this second approach, device-to-device (D2D) communication has been studied extensively, where nearby users can transmit data directly to each other with reused cellular resource blocks. Due to its local communication nature, D2D communication can be provided with smaller fees compared to the fees for cellular communication. D2D communication provides many benefits that cannot be provided by uncoordinated communication.
Some advantages of using D2D communication are summarized as follows:                The overall network spectral efficiency can be improved significantly with an optimal configuration;        Low delay and low power consumption due to the proximity of users;        Improving radio resource utilization because of resource reuse by both cellular users and D2D pairs simultaneously;        Using one link for direct communication, instead of one uplink and one downlink for communication through the base station, reduces resource usage; and        Offloading cellular traffic to D2D traffic reduces congestion in the backhaul network, benefiting cellular existing users in the network.        
There are many current and prospective applications for D2D communications. For example, D2D has been used in LTE-based public safety networks in the United States for its security and reliability. In addition, D2D communication is necessary for scenarios where the cellular transmission is not accessible.
In commercial networks, many social network applications require recognition of nearby users. Proximity user recognition is usually handled in a centralized manner, where users are required to register their location information in a server such that the location information can be shared among the other users, e.g., in the FACEBOOK. With D2D, location registration is no longer required for the purpose of proximity discovery. Another prospective application for D2D communication is E-commerce, where nearby agencies need to transfer efficiently a large amount of private data.
One challenge of D2D communication is interference to the coexisting cellular users. For a D2D underlying cellular network, interference needs to be carefully controlled because cellular users and D2D users share the spectrum. In order to manage the interference to the cellular users in the same cell, several approaches have been proposed such as limiting D2D transmission power, employing a fixed booster factor and a back-off factor to adjust D2D power. An interference limited area has been proposed, where D2D users can share the resources of those cellular users located out of the area.
In practical multi-cell networks, inter-cell interference (ICI) is a challenge that has not been addressed in the D2D literature. The ICI depends on the duplexing scheme used by cellular and D2D users and the resources blocks shared between D2D and cellular users. It is important to set the cellular user and D2D powers in an intelligent manner such that the ICI in the neighboring cell does not exceed some required upper limit.
However, the existing solutions are not without problems. Relay beamforming is a simple and efficient technique in order to take advantage of spatial diversity provided by multiple antenna receivers. The state-of-the-art algorithm to find the optimum D2D and cellular user powers is designed for a single-antenna. With receiver beamforming, the received SINR can be improved significantly at the BS resulting in overall spectral efficiency improvement. D2D communication could cause large ICI in the neighboring cells. There is no power allocation algorithm described in the literature to limit ICI caused by the users in the desired cell to users in adjacent cells. The current known algorithms are not designed to:                Use a multi-antenna BS with optimal beamforming vector to improve SINR of the cellular user; and        Maximize the sum rate of the cellular user and D2D with a limit on the maximum ICI generated in the neighboring cell.        
The D2D communication may be bi-directional communication where both devices receive and transmit in the same or different resources. However D2D communication scenario may also comprise that of one of the devices transmits and the other one receives the signals. There may also exist a point-to-multipoint (e.g. multicast, broadcast) scenario in which case a plurality of devices receive signals from the same transmitting device. This scenario is particularly useful for emergency services or public safety operation to spread vital information to several devices in an affected area. The term D2D communication and D2D operation are interchangeably used.
Typically, devices operate under the supervision of radio access network with radio access nodes (e.g. base station). But in some scenarios the devices themselves establish direct communication constituting the radio access network without the intervention of the network infrastructure
In cellular network assisted device-to-device communications (or simply network assisted D2D communications), cellular wireless devices in the vicinity of each other can establish a direct radio link (D2D bearer). While cellular wireless devices communicate over the D2D “direct” bearer, they also maintain a cellular connection with their respective serving base station, such as an enhanced Node B (eNB). This direct link is interchangeably called as network (NW) link, D2D-NW link etc. The NW link is used for example resource assignment for D2D communication, maintenance of radio link quality of D2D communication link etc.
As such D2D communication is a promising feature that can potentially scale the capacity of the network. In a D2D communication scenario, two cellular wireless devices (for example, UEs) directly communicate with each other without having the payload traversed through the backhaul network.
Three example coverage scenarios for D2D communication have been defined.
In Coverage
In this coverage scenario, all communicating D2D cellular wireless devices 14 are within the network coverage. In this scenario, the D2D cellular wireless devices can receive signals from and/or transmit signals to at least one network node such as the base station 12. In this case, the D2D cellular wireless device can maintain a communication link with the network. The network in turn can ensure that the D2D communication does not cause unnecessary interference. In coverage is also interchangeably referred to as in-network (IN) coverage.
Out of Coverage
In this scenario, D2D cellular wireless devices communicating with each other are not under network node coverage. In this scenario the D2D cellular wireless devices cannot receive signals from and/or transmit signals to any of the network nodes. Typically the lack of coverage is due to complete absence of the network coverage in the vicinity of the D2D cellular wireless devices. However the lack of coverage may also due to insufficient resources in the network nodes to serve or manage the D2D cellular wireless devices. Therefore in this scenario the network cannot provide any assistance to the D2D cellular wireless devices. The out of coverage is also interchangeably referred to as out-of-network (OON) coverage.
Partial Coverage
In this scenario at least one communicating D2D cellular wireless device is within network coverage, and at least one other D2D cellular wireless device is not under network coverage, but is communicating with a D2D cellular wireless device that is under network coverage. As mentioned above, the D2D cellular wireless device not being under network coverage can be due to lack of any network node in its vicinity or due to insufficient resources in any of the network nodes in its vicinity. The partial coverage is also interchangeably called partial-network (PN) coverage.
Establishing direct communication between two nodes, or even among a set of nearby nodes in Long Term Evolution (LTE) networks, is a promising way to enhance the spectral efficiency of the cellular network. Achieving potential improvements of Device-to-Device (D2D) communication depends on efficiently addressing the resource and power allocation problems. However, proposed solutions to these problems are immature. In a realistic cellular environment, there are multiple cellular devices and D2D pairs that attempt to access a shared resource pool. Normally each node has access to multiple Resource Blocks (RBs) and also each RB is allocated to multiple interfering devices between cells, i.e., resource reuse among neighboring cells. Furthermore, either uplink or downlink resources can be used for D2D communication. Any realistic D2D resource allocation formulation should consider the aforementioned factors. No existing work has addressed all of these factors. Studies in the literature propose simple heuristics that give highly sub-optimal performance, while the available optimal solutions are only achieved under simplified cellular communication models.
D2D communication can cause large ICI in the neighboring cells. The current algorithms available in the literature are not designed to maximize the combined sum rate of the cellular and D2D devices with a limit on the maximum ICI generated in the neighboring cell.
The D2D communication may be bi-directional communication where both devices receive and transmit in the same or different resources. However D2D communication may also comprise scenarios in which one of the devices transmits and the other one receives the signals. There may also exist a point-to-multipoint, e.g. multicast, broadcast, scenario in which case a plurality of devices receive signals from the same transmitting device. This scenario is particularly useful for emergency services or public safety operation to spread vital information to several devices in an affected area. The term D2D communication and D2D operation are interchangeably used herein.
Typically, devices operate under the supervision of a radio access network with radio access nodes, e.g., base stations. However, in some scenarios the devices themselves establish direct communication constituting the radio access network without the intervention of the network infrastructure.
Known studies propose simple heuristic methods that give highly sub-optimal performance, while the available optimal solutions are achieved under simplified cellular communication models. For example, in one known arrangement, an optimal resource allocation solution is provided for D2D devices underlying cellular devices in downlink transmission without imposing any constraint on the D2D power. In another known arrangement, a sub-optimal resource allocation solution is provided for D2D devices underlying cellular users in uplink transmission by dividing the original problem into three sub-problems and solving them separately. In another known arrangement, an assumption is made that each D2D pair has access to all RBs in its cell, without matching them with a specific cellular device. Further, some known proposed methods only consider a single-RB scenario or only consider a single-cell scenario. Some known proposed methods attempt to compute the power allocation for a D2D device and the corresponding cellular device at the same time. There are also known arrangements that use game theory approaches for resource allocation. These known works propose methods that do not ensure that any network related metric will be optimized.